Is an iPhone just as good for recording audio as the microphones that music producers use?
The microphones used in production vary: 1) Sound from all sides - Omnidirectional, 2) Sound from one direction - Cardioid, 3) Sound from two sides - Bi-directional
There's also different types of microphones, condenser, dynamic, ribbon, are the main types. All of them have different ranges, and all of them can only handle a certain amount of sound being thrown to them.
With the exception of the iPhone 5, all the iPhones have an omnidirectional microphone. The iPhone 5 has three, but they're all very narrow for clear/louder sound when you're speaking.
Like the others are saying, you want to invest into a proper microphone. The iPhone will record some things nicely, like a soft voice, and maybe acoustics to an extent, but it will mostly give you horrible, muffled, echoey sound otherwise.